Absorbent articles for personal hygiene of the type indicated above are designed to absorb and contain body exudates, in particular large quantity of urine. These absorbent articles comprise several layers providing different functions, for example a wearer-facing liquid permeable topsheet, a garment-facing liquid impermeable backsheet and in-between an absorbent core, among other layers. The function of the absorbent core is typically to absorb and retain the exudates for a prolonged amount of time, minimize re-wet to keep the wearer dry and avoid soiling of clothes or bed sheets.
The majority of currently marketed absorbent articles comprise as absorbent material a blend of comminuted wood pulp with superabsorbent polymers (SAP) in particulate form, also called absorbent gelling materials (AGM), see for example U.S. Pat. No. 5,151,092 (Buell). Cores having an absorbent material consisting essentially of SAP (so called “airfelt-free” cores) have also been proposed, see for example WO95/11652 (Tanzer), U.S. Pat. No. 6,790,798 (Suzuki), WO2008/155699 (Hundorf), or WO2012/052172 (Van Malderen). Absorbent cores with slits or grooves have also been proposed, typically to increase the fluid acquisition properties of the core or to act as a folding guide. WO2012/170778 (Rosati et al., see also WO2012/170779, WO2012/170781 and WO2012/170808) discloses absorbent structures that comprise superabsorbent polymers, optionally a cellulosic material, and at least a pair of substantially longitudinally extending channels.
The various components of an article are typically attached to another so that they stay in place before and during usage. Typical attachment means are gluing, heat and/or pressure bonding, ultrasonic bonding. The attachment means will be chosen by the manufacturer to balance costs of the equipment, cost of the glue material and performance required. Absorbent cores are typically attached to the backsheet by gluing, in particular by spraying the whole or most of the area between the core and the backsheet with a melt blow type adhesive. In presence of absorbent cores with slits or grooves, it may be beneficial to leave unglued areas between the absorbent core and backsheet components.
Alternative core-to-backsheet gluing methods have been proposed. WO2012/170341A1 (Hippe) discloses a diaper having a reduced core-to-backsheet gluing area. In Hippe, the absorbent core is attached to the backsheet only in certain, limited, areas to reduce the formation of buckles and wrinkles in the backsheet during usage, as well as the see-through of urine stains from the absorbent core through the backsheet. In particular, these unglued areas between absorbent core and backsheet are beneficial especially when the absorbent core comprises channels, which are areas substantially free of super absorbent material, and the core to backsheet attachment is provided only outside of the channels areas. This further improves the conformity of the absorbent article especially in loaded state.
However, when glued and unglued areas are present side to side between core and backsheet, boundary lines are inevitably created between the glued areas and the non-glued areas. The inventors observed that these boundary lines are the object of stress during the manufacturing of absorbent articles and can be at the origin of failures in the backsheet film. In particular when the boundary line is created by a discontinuous pattern of application of the glue (within the glue application area) like spirals or swirls this phenomenon is even more evident.
The backsheet materials for absorbent articles such as diapers are normally formed by a laminate of a very thin plastic film with a nonwoven where the nonwoven is oriented outside of the article (on the garment facing side) so to provide fluid barrier properties together with a soft touch. The two materials are called backsheet film and backsheet nonwoven respectively.
Thin plastic films are preferred as backsheet film because this reduces the environmental impact of the absorbent articles and increases the softness of the film, which is a very important feature for the consumers, however, in general, the thinner the backsheet film the lower its tensile strength. A low tensile strength increases the likelihood of failures during production or wear of the absorbent article. Moreover additional additives like Calcium Carbonate can be added to the backsheet film composition to produce porous backsheet films which enable water vapor transfer (also known as “breathable films”), and improve dryness of the skin during wear. Breathable backsheet films tend to have lower tensile strength and are in general more fragile than non-porous films of the same thickness. The lamination of the backsheet film with backsheet nonwoven enables to have a total tensile strength higher than the one of each individual layer. This lamination can be done for example with glue patterns like slots, spirals, or the like. Areas without glue between the backsheet film and backsheet nonwoven are desired to improve softness of the laminate, and the inventors have found that in those areas the backsheet film is more susceptible to fail and tear during manufacturing or wear of the absorbent article. Therefore a balance must be found between the film properties, the glue patterns used on the backsheet laminate between backsheet film and backsheet nonwoven, the glue distribution between the backsheet film and the core, each material tensile properties and the glue bonding strength to help withstand the stresses, especially on the boundary lines of the core to backsheet glue application area in high speed manufacturing.
Absorbent articles such as diapers are typically built on high speed lines which travel at speed that can be higher than 30 km/h, and at the end of the process, they are decelerated for folding and packaging from 30 km/h to 0 in a fraction of a second. This deceleration of the absorbent article is normally accomplished by nonmoving or low speed mechanical parts of the line, like plates, belts or fingers that holds the absorbent article by the outer layers, in particular the garment side of the backsheet. In the areas where core and backsheet are fully attached, the two components travel together and the kinetic energy of the product is dissipated along the full area in contact with the low speed or static parts of the production line. However the inventors observed that in areas where the core and backsheet are decoupled (i.e. not glued together), the absorbent core, which has a much higher mass and kinetic energy than the backsheet substrate, while decelerating releases a part of this energy on the boundary lines between glued and non-glued areas, so that backsheet film can tear along the attachment boundary lines. The inventors have also observed that in the areas where the backsheet is not fully glued to the nonwoven, small folds or pleats can be formed on the backsheet laminate.
The formation of folds or pleats on the backsheet negatively impact the quality appearance of the product, and, especially on the boundary lines separating glued and non glued areas between core and backsheet, can create “spot bonds” where the film stick to itself or to the absorbent core while folded. When the user of the absorbent article, e.g. of a diaper, picks it up from the package the article is normally folded. Before use the article is typically unfolded and stretched longitudinally, and stress can be concentrated on these spot bonds, and can originate tears on the backsheet, or the backsheet film.
Glues having a reduced bonding strength can of course reduce the occurrence of backsheet tearing, however the glue must ensure enough bonding strength between core and backsheet so to prevent collapse of the core once loaded with fluids. Also backsheet tearing could be reduced by increasing the thickness of the film or decreasing the manufacturing line speed, but both these solution are in general not acceptable as they reduce the efficiency of manufacturing and the environmental impact of the articles, also impacting manufacturing costs.
The present invention is directed to an improved core to backsheet gluing method and absorbent articles employing this improved gluing method. In the present invention the applicant has surprisingly identified a parameter range which provides a sufficient bonding strength while preventing the occurrence of backsheet rupture especially for absorbent articles with unglued areas between backsheet and core.